In communications networks, there may be a challenge to obtain good performance and capacity for a given communications protocol, its parameters and the physical environment in which the communications network is deployed.
For example, one parameter in providing good performance and capacity for a given communications protocol in a communications network is the ability of a wireless terminal to find a cell in the communications network to camp on. This is achieved by the wireless terminal performing a cell search. The cell search is typically performed when the wireless terminal is powered up, or after having been in so-called flight mode.
The wireless terminal preferably camps on a cell belonging to the correct network, its home public land mobile network (PLMN). When the wireless terminal starts its cell search for a cell in the communications network to camp on, it typically performs steps as summarized next. The wireless terminal performs a cell search using carrier frequencies stored in a history list. If no cell corresponding to these carrier frequencies is found, the wireless terminal continues its cell search by scanning the available frequency bands to find any cell. When a cell is found the wireless terminal reads the system information to check whether it is a home PLMN or otherwise if the wireless terminal anyway is allowed to camp on the cell. This procedure is continued until a cell is on which the wireless terminal is allowed to camp on is found.
There are many ways for the wireless terminal to scan the available frequency bands to find a cell. For wireless terminals supporting the Global System for Mobile Communications (GSM) and/or Wideband Code Division Multiple Access (WCDMA) based communications networks the wireless terminal performs measurements in those frequency bands where energy is received. Then the wireless terminal searches at every defined carrier frequency when the received energy (such as the received signal strength indicator, RSSI) is higher than a threshold.
With wireless terminals also supporting Long-Term Evolution (LTE) based communications networks need to support more and wider frequency bands, the above summarized cell search procedure becomes lengthy in time. For example, LTE Band 1 contains almost 600 carrier frequencies. If each search takes 1 second, this band and RAT alone takes many minutes.
It has therefore been suggested, see EP2351431 and EP2066043, that the wireless terminal is configured to search for a cell with a nominal spectral shape in the frequency band. In general terms, the spectral shape may be unique for each radio access technology (RAT). That is, each RAT may have cells with its own spectral shape. For example, the wireless terminal may search for cells with the spectral shape of LTE and using different bandwidths (such as 1.4, 3, 5, 10, 15 and 20 MHz) in order to estimate carrier frequencies where the cell search should be performed. In general terms, EP2351431 and EP2066043 propose to during the cell search use a filter (similar to a matched filter) adapted to the spectral shape of the searched RAT and bandwidth. The proposed cell search efficiently finds cells where the spectra are easy to distinguish from each other. One such example is schematically illustrated in FIG. 7. FIG. 7 schematically illustrates a spectrum 71 comprising three LTE cells 72a, 72b, 72c each with 10 MHz bandwidth and placed 10 MHz apart. Here the cell search procedure as disclosed in EP2351431 and EP2066043 works fine and identifies the three carries 73a, 73b, 73c (illustrated as peaks 74a, 74b, 74c having a probability close to 1), where three LTE cells with a bandwidth of 10 MHz and a nominal distance of 10 MHz are shown with center frequency locations 75a, 75b, 75c. 
However, when the carriers are pushed closer to each other, e.g. to 9.0 MHz so there is no gap at all between the spectra of the three different cells, as schematically illustrated in the example of FIG. 8, where center frequency locations 85a, 85b, 85c of three 10 MHz cells are shown, the cell search procedure as disclosed EP2351431 and EP2066043 interprets the spectrum 81 as comprising one continuous 30 MHz cell and thus finds only one potential cell in the frequency band 82 but fails to correctly find any center frequency locations, and thus fails to find a cell to camp on. One potential carrier 83 is found, but this carrier is determined to have a probability close to zero, as illustrated by the peak 84. In more detail, the cell search procedure detects that there is power, but in this case determines the frequency band 82 to be a continuous 30 MHz spectra, hence not corresponding to a nominal spectral shape and thus no cell to camp on is found.
Furthermore the spectra of the different cells may overlap even more in frequency, causing the received spectra to deviate even more from the nominal shape. Additionally, with multipath propagation the spectrum is corrupted even further, thus making the situation even worse.
Hence, there is still a need for an improved cell search in a communications network.